New Gas boiler - advice please

I would say from the customers I have asked and from my own experience that 25-30% is achievable but will require best case/worst case upgrade to acheive.

I.e. previously: permananent pilot, conventional flue, simple time switch no seperate control of HW, plain tank with tatty jacket.

afterwards: condensing boiler: Programmable thermostat, Part L compliant tank for S-plane controls and TRVs. and full control of HW.

Have a fan assisted boiler or electronic ignition in the old kit and the savings will be much less.

I'm talking mainstream domestic condensing system boilers that would be used for replacement in a smallish system. Yes they modulate just like combis have for a few years but your explanation is misleading. Fixed pump speed is the norm in the smaller boilers. The set point is the same old temperature limit as in older boilers, a limit which Potterton for example says set to max in winter. The modulation can only try to keep the temperature at the setpoint, not down to a lower setpoint. The boiler has to achieve the heat requirement of the radiators or heating effectivness is poor. The radiator system may reduce the flow as a result if TRV action. Perhaps that's why some condenser advocates now recommend removal of TRVs. A flow reduction causes the return temperature to rise, the modulation only goes so low, typically 50% of rating. Crucially in a system boiler the setpoint has to be high enough for safe DHW. In the mainstream domestic system boilers there is not currently a separate control loop for DHW. Yes if there was things would be better.

I'm not arguing against bespoke condensing design, only the dubious economics of compulsory replacement in some small systems.

Well it was a very simple response which assumed zero installation cost. There is a comment in the installer magazine at the merchants that if the rules had allowed 20% exceptions unscrupulous installers would have used this to undercut. I think that is a rounabout way of saying condenser installation is more expensive.

Ideally but spares are only guaranteed for ten.

I have to agree with that statement.

Jim A

Not really.

True on many system boilers, but a number even in smaller capacity have at least a switched pump.

If a self adjusting pump such as the Alpha is used then variation as a result of TRV behaviour is accounted for.

This depends on the season, the system and what one is trying to achieve. I wouldn't take anything from Potterton as indicative of anything. They are living on former glory and not a lot more.

On a conventional boiler, you should not really be turning the boiler thermostat down below the 82/70 figure too much since it will induce condensing and in this environment that is deleterious.

In a condensing boiler, the boiler thermostat can be turned down when the weather is warmer. This is because the heat loss from the house is lower and required radiator output lower. It's a fallacy to suggest that the system needs to be run at 82/70 for most of the time unless the radiators are marginally sized or the weather exceptionally cold.

It's true that more sophisticated condensing models have weather compensation and take this into account to control the water temperature, but this does not mean that the manual control that can be applied to a simpler condensing boiler to operate it in a lower temperature range rather than having it cycle is not valuable.

Actually not. If you look at products from Vaillant at the low capacity end they will modulate from 4.8 to 14.2kW - essentially 3:1 The small Worcester Bosch is similar.

Since the issue is one of compulsion through legislation, there are more issues to consider than the cost of your gas bill.

I suppose that a max power level could have been set below which fitting of condensing boilers was not mandatory (notwithstanding the other exemptions), but it's then adding another level of complexity to the whole thing. Since condensing combis, for right or wrong have a significant market share and in general have a minimum power level of

Not really, normally building regs only apply to new work or to modifications. This is an example of a like for like replacement being virtually prohibited at the component level regardless of system implications. IMHO that's going too far. Actually I understand that compulsory condensing is not going to apply in Scotland so my fear of not being able to obtain a conventional system boiler may not apply.

Jim A

It is for other replacements as well, such as windows (unless you go for other than elemental method for part L1 purposes)

This is from the government that brought you part P when that is demonstrably unnecessary and unenforceable.

There are other parts of L1 like having a token number of electrical light fittings that will only accept low energy light bulbs.

If you look at the Regulatory Impact Assessments around part L1, it is mainly around how to achieve carbon emission reduction and measures for that. Return on investment to householders does not figure highly in it.

It's difficult to guess at how prices of condensing and non-condensing boilers will move after April. It is alleged that there is a higher manufacturing cost in condensing models , but unless you are comparing against really simple conventional boilers with mechanical thermostat, there is not a huge difference in content apart from the arrangement to remove condensate - perhaps a fiver's worth of plastic bits at cost. So one could reasonably conclude that there could be cost differences that are volume related - i.e. as conventional boiler numbers decline they become more expensive to manufacture and condensing ones cheaper with increasing volume. It is hard to assess how amortisation of development costs will be treated - in other words whether non-condensers can be considered to have had that written down if they have been on the market for a while. Another unknown is how manufacturers will price and take margin on products, although that's more of a marketing, distribution and competition issue. It's possible that prices for condensing units will stay high in the short term, but I suspect that competition will do something about that ultimately. At any rate, I would expect that price differentials between condensing and non condensing will erode or disappear.

It's a pretty safe bet that energy prices will increase, so any percentage saving on use will have a direct impact.

There's not a great deal more that can be done to improve condensing boiler efficiency further with today's technology, which is why most units are grouped around the SEDBUK 90-91% range - and differences are not statistically significant. Therefore, the energy use gain, accounting for property size and type is fairly predictable or calculable.

THese seem to me to be the three economic factors and then one has to consider the time period.

I don't think that these are the main focus of the government in the legislation. They are assuming also that you will want to be sufficiently public spirited, but just in case you are not, they will help you with legislation which is for your own good :-)